Certain structures are produced by exploiting the superplasticity properties of certain alloys. Two plates of these alloys are superimposed and placed between two moulds, one of said moulds comprising cavities with a specific shape. The moulds are pressed together and a suitable device is able to inject gas under pressure between the plates, whose material yields in the cavities until their contour is married whilst being diffusion-welded to the other plate in front of the other portions of the moulds. However, it is necessary to cover one of the plates with a deposit of yttrium oxide or another substance having the property of resisting diffusion welding at the locations where this welding is not desired and especially in front of the cavities. One method of this type is illustrated by the U.S. Pat. No. 4,220,276. However, the deposit in practice is plasma-projected over the entire surface after a mask has been placed at the locations where the deposit needs to be withdrawn, the mask then being pulled up from the surface with the portion of the deposit which covers it. But the placing and preparation of the mask, especially when cutting it at the desired locations, takes up time and there is a risk that the mask may be damaged when cutting it.
The invention concerns a method to obtain an identical product whose main feature is the use of an under-pressure liquid jet for pulling up the coating at the locations it touches without inserting a mask.
The use of under-pressure liquid jets for removing material is already well-known to experts in this field. It normally concerns jets bearing abrasive particles, garnet for example, which cut plates, sheets or similar products. The application is somewhat different as in this instance it is desired not to touch the sheet itself, which generally requires that no abrasive is used in the liquid, but there is nevertheless a conventional problem of these machinings where the contour of the material removed is jaggy and lacks cleanness. In the current application, the result is that the shape of the portions of the plates yielding in the cavities, which shall generally constitute stiffening ribs, is not sufficiently regular, which may have a negative effect on the mechanical behaviour of the structure finally obtained.